A cataract is an opacity that develops in the crystalline lens of the eye or in its envelope. One medical procedure to remove a cataract-affected lens is phacoemulsification (phaco) using ultrasonic sound to break up or emulsify the cataract. A phacoemulsification machine typically includes a handpiece with both irrigation and aspiration functions. A phaco handpiece aspirates in emulsified fluids and simultaneously replaces those aspired fluids with balanced salt solution (BSS) to maintain a proper pressure of the anterior chamber of the patient's eye. Such a handpiece is connected to a pump generating negative pressure or vacuum to drive aspiration, by which debris from the eye flow through a tube to means for collection such as a cassette, a bag, or a bottle.
A common and potentially dangerous occurrence in ophthalmic surgery is “post-occlusion surge.” During ophthalmic surgery, particularly cataract surgery, as the lens is broken-up and emulsified, such as during phacoemulsification, irrigation fluid is constantly infused into the surgical site and the fluid and emulsified tissue are aspirated away from the surgical site through the phaco handpiece. On occasion bits of tissue are larger than the aspiration lumen in the phaco handpiece, which can result in a clogged phaco needle. As long as the aspiration conduit remains clogged, a negative pressure builds up throughout the aspiration system. Then, after the clog has been removed, the system can experience what is commonly referred to as surge. Post-occlusion surge can cause serious damage to a patient's eye, such as by rupturing a capsular bag and allowing vitreous to leak from the eye's posterior into the eye's anterior chamber or cause irreparable damage to the cornea's endothelial cells. Generally speaking, endothelial cells are not regenerated naturally and it is crucial to prevent post-occlusion surge in an ophthalmic operation.
Air evacuating pumps, such as a rotary vane pump and a venturi pump, are widely used as sources of vacuum for surgical aspiration. In the case of a rotary pump, for example, spinning of the rotor is necessary to force fluid to move through a tube to a reservoir where the spinning of the rotor generates vacuum to drive aspiration. One way to reduce post-occlusion surge is to sense an occlusion at the tip of the handpiece or the conduit and direct the pump to stop so that vacuum is not generated any more.
Another approach is to monitor and control the fluidic pressure automatically to reduce excessive negative pressure. For example, U.S. Pat. No. 3,902,495 describes a control system containing a relief valve discharging undue pressure from the tube upon reaching a predetermined pressure.
However, the methods mentioned above and other conventional pressure controlling methods failed to recognize or solve the problem that disconnection of an energy source, such as turning off the vacuum pump, or introducing a relief valve does not lead to an immediate stop because the pump has momentum to continue its motion. Such momentum continues to generate negative pressure notwithstanding the disconnection of the energy source of the pump, resulting in a delay in ceasing the generation at a negative pressure and the stop signal. Given that the anterior chamber is considerably small in volume, the delay in stopping the pump can cause damage to the eye. Therefore, there is a need to eliminate or reduce such an undesirable effect arising from momentum-driven pumping in ophthalmic surgery.